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This is a small crawl space of less than 100 ft^2, and about half of the dirt floor is covered in a plastic sheet. I'm not sure whether the plastic sheet helps or not. The crawlspace has a very low ceiling such that I can't physically get at all of it because I don't fit, so encapsulating it is not feasible without digging it out. It has one vent and has been at 70%-80% humidity in the week since I started measuring, although it has also been rainy so maybe the situation would be improved in drier weather. I would rather not use a fan or make another vent because I am concerned about heat loss.

Of course, the recommendation is to encapsulate the crawlspace and only then put a dehumidifier in. But if I don't do that, will the dehumidifier help at all, or will it simply be overwhelmed by the rate of moisture transfer from the soil?

causative
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2 Answers2

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To answer your question directly, we have no idea. The RH may be lower than the ~100% I'd expect from damp soil due to air exchange with the exterior, or it may be that the soil is fairly dry and the evaporation rate is low. If the soil is damp, no consumer-grade unit will keep up. If it's dry, it'll reduce RH somewhat, but at a cost.

That said, it's likely that a dehumidifier will run a lot regardless. Knowing how fragile they are, and how much energy they consume, I'd certainly attempt to resolve the problem with airflow to the outside instead of a powered device. If the air temperature in your crawl space was to get so low that it's a heating concern, you probably don't have a moisture damage concern. Not much happens in cold temps regardless of RH.

In an ideal world your floor is insulated and your crawl space is ventilated unconditioned space (the latter at least during warm months if pipe freeze is a concern).

isherwood
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will the dehumidifier help at all,

The dehumidifier may help but the only way to know is to try it.

I'm not sure whether the plastic sheet helps or not.

Probably not much. To be effective it should be at least 10 mil and cover all of the soil, extending up the foundation walls at least 6 inches and sealed to the foundation.

I recently managed a similar project here in Colorado, which has lower humidity levels than most of the country so there may be some differences with your situation.
High humidity can cause problems with proliferation of molds, specifically in walls and other enclosed spaces although I doubt that it could rot the floor joists. Regardless, it's a good idea to do what you can to mitigate the problem.
There are a number of variables involved with your crawl space that you don't address in your post.

  • The water source, which could be surface water entering through the foundation or a subterranean spring forcing water to the surface beneath your home - who knows?.
  • How much water is entering the crawl space (flow rate).
  • Type of foundation.
  • Vapor barrier, insulation on foundation walls, etc.

On the Colorado project I brought in a soil engineer who found that the water was ground flow coming in under the foundation wall which was set on piers. The solution was a perimeter drain and sump pump in the crawl space along with some other changes. Our goal was to bring the RH down to 50, which it accomplished. Based on the info in your post you are much more limited on crawlspace access and easy options.

My suggestion is to do the easier, less costly, things first.
That is: eliminate/reduce the possibility of surface water causing the problem.

  • Make sure that grading around the home is carrying water away from the foundation. A 4 foot perimeter is recommended.
  • All downspouts should have, at least, 4 foot extensions to carry rainwater away from the foundation.
  • Lawn sprinklers should also be adjusted to minimize over spray on or around the foundation.

If this doesn't help it you might consider bringing in an energy efficiency professional to evaluate your situation.

HoneyDo
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